Massachusetts Comprehensive Assessment System

2011, High School Intro Physics

Reporting Category: ElectromagnetismStandard: 5.2 - Develop a qualitative and quantitative understanding of current, voltage, resistance, and the connections among them (Ohm's law).

An electrician places a variable resistor in series with an LED lamp and a 6 V battery, as shown below.

LED lamps are efficient sources of light that operate at a specified voltage for a wide range of currents. The LED lamp in this circuit operates at a constant 3 V for all settings of the variable resistor, while the remaining 3 V is always dropped across the variable resistor.

Calculate the current passing through the variable resistor when it is set to each of the values shown in the table below. Show your calculations and include units in your answers.

Voltage (V)

Resistance (O)

Current (A)

3.0

4

3.0

6

3.0

10

3.0

15

3.0

20

On the grid in your Student Answer Booklet, make a graph using your answers to part (a). Current should be on the y-axis and resistance should be on the x-axis. Clearly label the axes and use a proper scale.

Based on the graph, describe what happens to the LED lamp as the resistance decreases.

The maximum recommended current for the LED lamp is 0.3 A.

If the LED lamp is operated for a long time with the variable resistor set at 4 O, describe the most likely effect on the LED lamp.

Scoring Guide and Sample Student WorkSelect a score point in the table below to view the sample student response.

Response demonstrates a thorough understanding of the relationships between current, voltage, and resistance. The response correctly calculates the current values for each resistor value and correctly graphs current and resistance. The response also correctly describes what happens to the LED lamp as the resistance decreases and if the resistance is set at 4 Ω for a long time.